Scamper needs socialised in my opinion. I don't believe in closeting dogs because of their insecturities. Some people say I'm mean but it's not natural for a dog to be scared of the out doors. Following your vets advice won't solve the problem and IMO will compound it.

If he's fine off lead then he sees the leash as a control device..which it is!
He doesn't like it and want's to do his own thing. But that's not good enough. the pack re structuring inside the house will help over come this. Also I suggest you keep moving. Sounds silly but when we turn and show atention to them when they are in a timid state we empower and cement their behaviour. So try to keep moving forward if he drops down then turn back lift his back end up and try to get him on his front paws. once his done this stand him up and move off again. It's a case of conditioning the mind to stay on all fours. Try titbits that might work too, again follow the rule of might get one might not. You want your dog doing this for you not for the treat.

Try running with him in spurts then stopping and running off again. To get his mind thinking different. Dogs tend to have difficulty thinking of more than one thing at atime. So keep moving forward ,use titbits to encourage forward movement.

The walks very important too. How we walk with our dogs effects their behaviour.

I suggest:

You sit with the lead and call him over give him a titbit and slide the leash on. let him wander off for a while then recall him. pick up the leash and move to the door. make him sit calmly by the door. Don't move outside until he is. Open the door and keeping him behind you step outside leading him behind you, don't look at him though. when outside make him sit again. give him calm affection or titbit then move off. Keep him at your side. Although he's timid i wouldn't let him rule the walk so don't let him get infront. don't let him wander ,sniff or make any choices at the moment. Move quickly as you can engaging his attention. This might work it's hard to say not seeing him. Remember if you expect him to behave in a certain way he will. You and your dog need to leave the past behind to move foward.